Behaviour of HMC733 that seem to be out-of-specs

Hallo everyone!

We are having problems with HMC733 in our synthesizers. When the Vtune voltage is at a level corresponding to fundamental freq close to 18 GHz we observe that the VCO generates a particularly strong sub-harmonic signal at ca. 9 GHz This could be even stronger that the fundamental. Needless to say that VCO behaving like this baffles the PLL completely.

A number of HMC733 parts installed in a PLL-based synthesizer develops this strange behaviour either right after installation or after a short while. As a result the synthesizer is unable to generate any reasonable signal close to 18 GHz. Besides this sub-band all seems to work as expected.

To check what is going on we have installed one of the problematic VCOs on a bare PCB with the Vcc, and Vtune connected and the Pout routed with a semi-rigid coaxial line. With this circuit we could observe the behaviour of the VCO only and confirm that a number of chips we were working with exhibited the behaviour like described above. The problem is unlikely to be connected to the mounting/soldering procedure of our chips. Over the last year we built a number of synthesizers using the same soldering techniques and ...all of them are still working correctly. A month ago we built a synthesizer (let's call it SythA) and it also works OK. Using the same PCB layouts, and the same components we built  then a copy of it -- SythB. All the problems described above were observed in SythB.

I wonder what operating conditions can cause HMC733 to work in a way described above. We have seen this behaviour in at least 3 chips now. The last one is quite interesting... We had a EVAL board EV-ADF41513SD2Z (with a HMC733 installed). It worked fine so we took its HMC733 out, checked that the harmonics are below -20 dBc and all is within specs. Then we installed it on SynthB, hoping that a verified and good component will bring SythB back to life in all 10-20 GHz band. This helped as the synthesizer was working fine for a week or so. But... after this week we are now seeing again the behaviour described in the beginning of this post quite often. Again, the synthesizer does not work close to 18 GHz, while it works fine outside of this band.

We will appreciate a lot some hints as to what can go this wrong in only one of the sythesizers (SynthB) while the another one (SynthA) working exactly in the same condiftions does not show any problems.